As an example, iron ore which has been separated from the earthen substances with which it occurs in the mine, is commonly prepared for feed to a blast furnace by forming it into green pellets then feeding the pellets so formed onto a traveling grate machine where the pellets are dried, preheated, and indurated at a high temperature. In some cases the pellets may then be transferred to some other device such as a rotary kiln and then cooled. The present invention desirably makes use of a process and apparatus as disclosed in Phelps & Anthes U.S. Pat. No. 3,172,754 dated Mar. 9, 1965 with slight modifications.
As is common in apparatus of this kind, the traveling grate is made up of a succession of chain castings or pallets assembled into a continuous conveyor, which moves horizontally through a series of zones made up of furnace chambers above the grate chain and a succession of windbox chambers beneath the grate's chain. The traveling grate chain travels through the furnace chambers then over a head shaft then under the respective windboxes through ambient air then back to its starting point. At the discharge end of the grate the pellets can be fed into a rotary kiln for additional heating or the pellets can be cooled by air being forced upwardly through the bed and discharged into a bin for storage.
While I specifically referred to a pelletizing ore method, essentially the same procedure is followed where the ore is supplied onto the traveling grate chain without being pelletized and this invention is applicable to heat induration by other processes such as sintering.
The process of this invention has, as a starting material, finely ground iron ore concentrates or other fines such as ore dust in a moist condition such as are generally produced by known iron ore benification processes or particularly magnetic concentrations of magnetite and flotation of non-magnetic ores.
The finely ground, moist ore concentrates are readily formed into small agglomerates or green water bound pellets by such apparatus as bailing drums and pelletizing discs fully described in U.S. Pat. No. 1,994,718. The pelletization is controlled so it can produce small, ball-like green pellets in sizes ranging from one-quarter of an inch to one inch in diameter. It has been found that a layer of about eight to thirty inches deep of pellets on the grate will give good results if the pellets average in size between three-eights and three-quarters of an inch in diameter.
The bed of green pellets formed in the previous step are placed on the traveling grate chain as it enters the first furnace chamber and are initially subjected to an up or down draft of moderately heated drying gas such as air. There may be a second drying chamber through which the still moist pellets are conveyed. In the next furnace chamber the pellets are subjected to a downdraft of flame heated gas in the neighborhood of 1800 degrees fahrenheit. In this step of the process the pellets at the bottom of the bed can reach a temperature approximately 300 degrees fahrenheit. The bed of material on the traveling grate chain is subjected to additional heating in subsequent furnace chambers until the material is hardened to the proper degree and all other processing has been completed. At that point the pellets are deposited or discharged into a device such as a rotary kiln for further treatment, or cooled and deposited in a storage bin.
A limitation on the use of this type of a process is the temperature that the grate chain material can withstand before yielding and failure. As the grate chain progresses through the various furnace chambers, more and more heat is applied to the bed of pellets until the temperature of the grate chain approaches its limiting point and the process has to stop, or as in most commercial applications the ore is transferred off the grate for further treatment. This limits the length of the grate that can be used or the temperature of the gas in the furnace used for heating the ore. A typical method of this type is shown in U.S. Pat. No. 3,285,735 D. D. Phelps. Another example of pertinent prior art is U.S. Pat. No. 2,750,272 O. G. Lellep.